Oil reservoirs generally exist deep within the earth under layers of soil and rock. Once an accumulation of oil has been located, a series of wells are drilled in a predetermined pattern to effectively drain the reservoir. The total amount of oil in a reservoir is often referred to as the amount of “original oil in place,” abbreviated OOIP. The first stage of recovery is referred to as “primary recovery.” In primary recovery, the natural reservoir pressure causes the oil to freely flow into the drilled production wells. However as time passes and the existing gas and water fully expand, the force exerted on the reservoir gradually dissipates and oil production decreases and ultimately stops. Primary recovery techniques are generally able to recover only about 10% to about 20% of the OOIP. After primary recovery methods have been exhausted, improved oil recovery methods must be used to recover additional OOIP. Secondary and tertiary methods can lead to a total recovery of up to about 50% of the total OOIP.
Secondary recovery often involves flooding the reservoir with water to force oil into the production wells. The water used in oil flooding must have a specific chemistry to not destabilize clay formations and render the soil surrounding the reservoir impermeable. The water must also be relatively low in salinity and nearly free of sulfate. Secondary recovery with water flooding can recover up to about an additional 20% to 30% of the OOIP.
Tertiary, also referred to as enhanced recovery, may involve flooding the well with water dosed with chemical agents. Tertiary recovery can result in an additional recovery of up to about 20% of the OOIP. In most cases, a substantial amount of oil is left in the reservoir even after tertiary treatment because the remaining OOIP cannot be recovered economically.
Improved oil recovery methods (i.e. secondary and tertiary methods) require a tremendous amount of water. It takes time to inject sufficient water to fill enough of the void spaces in the reservoir to move the OOIP. It takes several months from the start of a water flood before significant production increases take place and an average flood usually lasts from about six to about ten years.